Off on a comet hunt – for up-close view of icy rocks

Planetary scientists are set to write the opening chapter of A Tale of Two Comets with next Monday's launch of Contour, an unprecedented mission that will peer into the cores of two comets over the next four years.

Where ground-based astronomers can see only a fuzzy glowing ball whose core is shrouded in dust and gas, Contour will be able to pierce that veil – and spot details on the comet's nucleus only 12 feet across.

For millenniums, comets have captivated the human imagination. But only recently have they come within reach of science's virtual grasp – first through optical and radio telescopes, then via spacecraft.

The Contour mission, which planners hope will encounter its first comet in November 2003, promises to expand understand-ing of these galactic wanderers. Comets are the remnants of early planet-building in the outer solar system. Their collisions with a young Earth are thought to have supplied the planet with water and other chemical compounds necessary to give rise to organic life, and to have altered the course of evolution.

As a result, their significance to humans has shifted: from astrologers' omens of imperial doom to astronauts' depots for refueling rockets – at least potentially. "One day, these may be our filling stations and watering holes as we colonize the solar system," says Donald Yeomans, a senior research scientist at the Jet Propulsion Laboratory in Pasadena, Calif. So, he quips, the justification for launching Contour "has passed my 'brother-in-law test' " of relevancy.

Armed with Contour's observations, as well as with data from past comet flybys, researchers can begin to explore the diversity of comets arising at different stages of their lifecycle and found in different regions of the solar system. "This is an essential next step in exploring comets," says Joseph Veverka, headof Cornell University's astronomy department andContour's lead investigator.

Contour's camera is designed to spot details as small as 4 meters across, compared with 40 meters for Deep Space 1's flyby of comet Borrelli last year and 100 meters for Giotto's Halley flyby in 1986. It hosts a dust analyzer designed to return samples of cometary dust to Earth. Other Contour instruments will examine the chemical composition of material in the nucleus and tail.

Contour's targets, comets Encke and Schwassmann-Wachmann 3, are polar opposites. Schwassmann-Wachmann 3 is a solar-system newbie whose nucleus broke apart several years ago, giving Contour scientists a rare chance to examine the structure of a comet's nucleus, as well as its chemical makeup.

Encke is nearing the end of its career, but of the two, it may be the more perplexing. Discovered in 1786 and visible to the naked eye, the comet has been orbiting the sun once every 3.2 years for thousands of years – yet records are devoid of sightings prior to 1786. And despite its age, Encke is not going out quietly: It is emitting much more gas and dust than astronomers think it should, given the number of passes it's made around the sun.

Encke was also the comet that gave rise to the "dirty snowball" model of comet composition, which holds that these primitive objects are conglomerations of dust and ice. The idea was first posited by Harvard University astronomer Fred Whipple in 1950 and was confirmed when Europe's Giotto spacecraft flew by comet Halley in 1986.

Some scientists believe that a cast-off chunk of material from Encke is responsible for the explosion over Tunguska in Siberia in 1908. The blast flattened 830 square miles of Siberian forest and left an orange glow in the night sky seen as far away as Western Europe.

Both comets are thought to have originated in the Kuiper Belt, a collection of rocky, icy objects that orbit 30 to 50 times farther from the sun than Earth.

Another comet source, the Oort Cloud, surrounds the sun at a distance some 1,000 times father than the Kuiper Belt. By some estimates, the cloud hosts a trillion comets, which enter the solar system only when their orbits are disrupted by a passing star or molecular cloud.

Contour mission planners are aiming to encounter comet Encke on Nov. 12, 2003, then meet up with Schwassmann-Wachmann 3 on June 19, 2006. After that, they hope to redirect the craft to visit one of the longer-period Oort Cloud comets, should one enter the inner solar system with a favorable orbit.

Indeed, the $159-million mission represents a change in the way comet fly-bys are conducted, notes Robert Farquhar, mission director at the Johns Hopkins University's Applied Physics Laboratory.

In effect, the comets will come to the spacecraft, rather than sending the spacecraft to the comets. Contour will orbit the Earth and sun, enlisting their gravity to sling it past comets selected for their favorable orbits.

This "let 'em come to us" approach saves fuel, and could even allow mission planners to intercept a newly discovered comet.